The hepatitis B virus preS2 antigen gene linked in one contiguous reading frame to the hepatitis B virus surface antigen gene has been expressed in a transformed host. The expressed protein is useful as a vaccine for the treatment and prevention of hepatitis B virus-induced diseases and/or infections and in in vitro diagnostic systems. The purification of the expressed protein from either broken cell slurries or from cell supernatant liquids, however, has heretofore resulted in large losses in yield due to degradation of the antigen.